The present invention relates to an AC generator for use in a vehicle in which ventilation and cooling can be achieved by use of a fan of a bracket support type rotor and, in particular, to the cooling of an exciting coil included in the rotor.
In FIG. 2, there is shown a sectional view of a conventional AC generator for use in a vehicle. In this figure, a stator 1 includes a stator core 2 and a stator coil 3. A rotor 4 is constructed in the following manner: heteropolar magnetic pole cores 5 and 6 are formed such that their respective magnetic pole pieces are projected alternately along the circumference of the rotor 4. The magnetic pole cores 5, 6 are also formed in such a manner that, when the axial length thereof is expressed as L and the height thereof is expressed as H, each of them has a relation of L &gt;H. An exciting coil 7 is held between the two magnetic pole cores 5 and 6. A rotary shaft 8 connects fixedly the cores 5 and 6. A pair of slip rings 9 are fixed through respective insulation sleeves to the rotary shaft 8. Fans 10 and 11 are fixed to the magnetic pole cores 5 and 6, respectively.
Front and rear brackets 12 and 13 are fixed to the stator core 2 by means of a fastening bolt 14. The front bracket 12 includes a plurality of air suction holes 12a and a plurality of air exhaust holes 12b which are used for ventilation. Also, the rear bracket 13 similarly includes a plurality of air suction holes 13a and a plurality of air exhaust holes 13b respectively used for ventilation. The rotary shaft 8 is supported through bearings 15 and 16 by the front bracket 12 and rear bracket 13. A pulley 17 is fixed to the rotary shaft 8. When the rotation of an engine is transmitted thereto, then the pulley 17 rotates the rotary shaft 8.
A changes 18 rectifies an alternating current, which is generated due to an alternating voltage induced in the stator coil 3, to a direct current. A voltage regulator 19 detects a generator voltage and controls an exciting current so as to adjust a terminal voltage to a predetermined value. A brush holder 20 is used to hold a brush 21 and presses the brush 21 against the slip rings 9.
In the above AC generator, if the rotor 4 is rotated, then an alternating voltage is induced in the stator coil 3 and electric power is supplied, with the additional result that heat is generated. The, stator 1 and rotor 4 are cooled by means of ventilation caused by rotation of the fans 10 and 11.
The cooling air produced by means of the rotation of the fan 10, as shown by arrows in FIG. 2, is sucked in through the suction holes 12a in the front bracket 12, and is then discharged from the exhaust holes 12b so as to cool the magnetic pole core 5 and stator 1. The cooling air produced by the rotation of the fan 11, as shown by other arrows in FIG. 2, is sucked in through the suction holes 13a in the bracket 13, and is discharged from the exhaust holes 13b so as to cool the rectifier 18, voltage regulator 19, electricity collector (not shown) magnetic pole core 6, and stator 1.
As described above, in the conventional AC generator in a vehicle, the magnetic pole cores 5 and 6 are formed such that each of them has a relation of L &gt;H where L expresses the length thereof and H expresses the height thereof. As a result, the magnetic pole cores 5 and 6 have a limited heat transfer surface area in contact with the exciting coil 7. Therefore, the rotor 4 is incapable of efficiently transferring and diffusing heat from the exciting coil 7 to the magnetic pole cores 5 and 6, and the exciting coil 7 exhibits a high rate of temperature increase. Accordingly, the exciting coil 7 and suffers from such drawbacks as decreased life and reduced magnetomotive force, due to an increase of its electrical resistance